1. Field of the invention:
This invention relates to a semiconductor laser array device which oscillates high output power laser lights with a 0.degree.-phase shift therebetween.
2. Description of the prior art:
Semiconductor laser devices having a single lasing filament structure can only produce 50 mW lights at their best. In order to produce high output power, semiconductor laser devices, in which a plurality of lasing filaments are disposed in a parallel manner on a single substrate to achieve an optical coupling between the adjacent lasing filaments, have been studied.
In the case where a plurality of index-guided semiconductor laser devices are disposed in a parallel manner with an optical phase coupling between the adjacent lasing filaments and the individual laser devices have the same gain, they tend to oscillate laser lights with a 180.degree.-phase shift therebetween rather than with a 0.degree.-phase shift therebetween. This is because the electric field distribution is in accord with the gain distribution in the 180.degree.-phase shift mode rather than in the 0.degree.-phase shift mode, resulting in a small oscillation threshold gain in the 180.degree.-phase shift mode. In order to prevent this phenomenon, the oscillation threshold gain in the 180.degree.-phase shift mode must be increased to suppress the 180.degree.-phase shift mode. For this purpose, as shown in FIGS. 4(a) and 4(b), semiconductor laser arrays having a structure with branching and combining type waveguides 31 on both facets 30 have been proposed. Each waveguide 31 of the semiconductor laser array shown in FIG. 4(a) is provided with two branching portions 31a, and that of the semiconductor laser array shown in FIG. 4(b) is provided with one branching portion 31a. The basic operation of these laser array devices is as follows: Lights, which are propagated with a 0.degree.-phase shift therebetween in the branching waveguides, proceed to the combining waveguides with the same phase, so that the lights are intensified. On the contrary, when lights, which are propagated with a 180.degree.-phase shift therebetween in the branching waveguides, proceed to the combining waveguides, they exhibit a reciprocal phase with each other, so that the lights are considerably weakened, resulting in a radiant mode in which the lights radiate outside of the waveguide. In such a manner, the lights with a 180.degree.-phase shift therebetween undergo loss, resulting in an increase in the oscillation threshold gain in the 180.degree.-phase shift mode.
However, oscillation threshold gain in the medial mode between the 0.degree.-phase shift mode and the 180.degree.-phase shift mode is reduced with an increase in the number of waveguides, so that oscillation in the 0.degree.-phase shift mode cannot be achieved. For example, FIGS. 5(a) to 5(c) show the distributions of electric field E and the far-field pattern with regard to the 0.degree.-phase shift mode (a), the 180.degree.-phase shift mode (c), and the medial modes (b) therebetween, respectively, in a laser array having a structure with a plurality of parallel active waveguides shown in FIGS. 5(a) to 5(c). It can be seen from FIGS. 5(a) to 5(c) that the far-field pattern with regard to a 0.degree.-phase shift mode (a) exhibits an emission direction of laser lights which is vertical to the emitting face of the array device, whereas the far-field patterns with regard to the other modes (b) and (c) exhibit the emission direction of laser lights with an inclination to the emitting face of the array device. That is, the electric field distribution of laser lights in the 0.degree.-phase shift mode exhibits peaks in the center, while the electric field distributions of laser lights in the other modes (b) and (c) exhibit peaks on both sides.
The far-field pattern of laser lights with a 0.degree.-phase shift therebetween produced by conventional semiconductor laser array devices exhibits a single peak so that the laser lights can be concentrated into a spot by means of optical lenses, whereas the far-field pattern of laser lights with a 180.degree.-phase shift therebetween or each of the medial modes between the 0.degree.-phase shift mode and the 180.degree.-phase shift mode exhibits dual peaks so that the laser lights cannot be concentrated into a spot by any optical lens. These semiconductor laser array devices producing laser lights which cannot be concentrated into a spot are inconvenient in optical coupling with other optical systems, and cannot be used as light sources for optical communication, optical disc systems, etc.
Thus, creation of a semiconductor laser array device oscillating laser lights with a 0.degree.-phase shift therebetween (i.e., attaining higher oscillation gain in the 0.degree.-phase shift mode than in the 180.degree.-phase shift mode and in the medial modes), which can thereby be utilized as a light source for optical communication, is required.